Fix WebKit2 build with newer clang compiler

Fixes the following build error:

    In file included from DerivedSources/WebKit2/WebVideoFullscreenManagerProxyMessageReceiver.cpp:32:
    Source/WebKit2/Platform/IPC/HandleMessage.h:16:25: error: non-const lvalue reference to type 'Vector<[3 * ...]>' cannot bind to a temporary of type 'Vector<[3 * ...]>'
        (object->*function)(std::get<ArgsIndex>(std::forward<ArgsTuple>(args))...);
                            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    Source/WebKit2/Platform/IPC/HandleMessage.h:22:5: note: in instantiation of function template specialization 'IPC::callMemberFunctionImpl<WebKit::WebVideoFullscreenManagerProxy, void (WebKit::WebVideoFullscreenManagerProxy::*)(WTF::Vector<std::__1::pair<double, double>, 0, WTF::CrashOnOverflow> &), std::__1::tuple<WTF::Vector<std::__1::pair<double, double>, 0, WTF::CrashOnOverflow> >, 0>' requested here
        callMemberFunctionImpl(object, function, std::forward<ArgsTuple>(args), ArgsIndicies());
        ^
    Source/WebKit2/Platform/IPC/HandleMessage.h:120:5: note: in instantiation of function template specialization 'IPC::callMemberFunction<WebKit::WebVideoFullscreenManagerProxy, void (WebKit::WebVideoFullscreenManagerProxy::*)(WTF::Vector<std::__1::pair<double, double>, 0, WTF::CrashOnOverflow> &), std::__1::tuple<WTF::Vector<std::__1::pair<double, double>, 0, WTF::CrashOnOverflow> >, std::make_index_sequence<1> >' requested here
        callMemberFunction(WTF::move(arguments), object, function);
        ^
    DerivedSources/WebKit2/WebVideoFullscreenManagerProxyMessageReceiver.cpp:55:14: note: in instantiation of function template specialization 'IPC::handleMessage<Messages::WebVideoFullscreenManagerProxy::SetSeekableRangesVector, WebKit::WebVideoFullscreenManagerProxy, void (WebKit::WebVideoFullscreenManagerProxy::*)(WTF::Vector<std::__1::pair<double, double>, 0, WTF::CrashOnOverflow> &)>' requested here
            IPC::handleMessage<Messages::WebVideoFullscreenManagerProxy::SetSeekableRangesVector>(decoder, this, &WebVideoFullscreenManagerProxy::setSeekableRangesVector);
                 ^

* UIProcess/ios/WebVideoFullscreenManagerProxy.h:
* UIProcess/ios/WebVideoFullscreenManagerProxy.mm:
(WebKit::WebVideoFullscreenManagerProxy::setSeekableRangesVector):
Make Vector<std::pair<double, double>>& argument const because a
temporary is not allowed to bind to a non-const reference.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@177624 268f45cc-cd09-0410-ab3c-d52691b4dbfc